An injection unit of this type is known, for example, from DE 43 44 335 C2 or DE 42 06 966 A1.
The document mentioned first discloses an injection assembly for an injection moulding machine, in which the plasticizing screw, which is axially and rotatably displaceable in the screw cylinder, is driven by a drive unit, which is based on a two-motor solution. Two hollow shaft motors arranged coaxially one behind the other are provided, one of which acts on the spindle nut of a spindle drive for the axial movement of the screw. The other hollow shaft motor sits on the hub of a spline shaft arrangement, the spline shaft of which is non-rotatably, but axially displaceably connected to the spindle of the spindle drive of the screw. Thus, the plasticizing screw can be made to rotate. If the two motors operate at the same rotational speed and in the same rotational direction, the screw merely carries out a rotational movement but in the case of rotational speed differences between the two drives, for example if the rotary drive is at a standstill and the spindle motor is activated, the screw is merely displaced axially, for example, for the injection movement.
The drawback in two-motor systems of this type is the fact that, apart from the actual motors, a frequency converter is also provided in each case for the rotational speed and torque regulation for each motor. Furthermore, the motors and the associated frequency converters have to be designed for the necessary maximum power taking into account the so-called ED factor (average operating period per hour). A combination of the power of two motors if the power thereof is not required, is not possible to realise a function of the injection unit.
One approach for circumventing the above problems is a so-called “one-motor solution” as shown in the published application DE 42 06 966 A1 mentioned second. A single drive motor is connected there by means of a belt drive to a spline shaft, which is non-rotatably but axially displaceably coupled to a spindle drive. The individual screw functions are controlled by means of a controllable coupling between the rotatably mounted spindle nut and the housing of the injection unit. With an engaging coupling, the spindle nut is blocked with respect to its rotatability, so that when the drive is rotating, an axial displacement of the screw takes place with simultaneous rotational movement. When the coupling is not engaged, the spindle nut rotates together with the spindle, so that only a rotational movement, but no axial movement of the screw takes place.
This known injection unit does have a compact drive unit with only one motor and correspondingly only one frequency converter but the fact is disadvantageous that the axial and rotational movement of the screw cannot be decoupled. This may be acceptable for the purpose of use specifically provided in DE 42 06 966 A1 for use in packaging machines but not for injection units of technically demanding injection moulding machines, with which high production outputs and workpiece qualities are to be achieved.